During processing of silver halide photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions which are more compatible with the environment, and thus it is desirable to reduce or avoid the use of ferric ions and many of the common complexing ligands.
Peracid bleaching solutions, such as those containing peroxide, persulfate, perborate, perphosphate, perhalogen, percarboxylic acid or percarbonate bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products can be less harmful.
However, there are certain problems associated with peroxide bleaching solutions. For example, acidic peroxide bleaching solutions are described in US-A-4,277,556 (Koboshi et al) which may contain metal ions as catalysts to improve the bleaching efficiency of the peroxide. Some of these solutions are stable, but they are not rehalogenating or silver retentive (that is, the developed silver can be oxidized to a soluble salt, and undeveloped silver halide remains in the element). This complicates silver recovery. Other peroxide bleaching solutions are known which contain a ferric ion complex as a catalyst. Such solutions are rehalogenating, but lack stability as iron is a well known catalyst for peroxide decomposition.
Thiols have been described as peroxide bleaching catalysts in some literature (JP-A-61/261739), but no demonstration of their effect was provided. Nonetheless, thiols are undesirable as catalysts because of their objectionable odors.
EPA-0 428 101 describes alkaline peroxide bleaching solutions which are rehalogenating, but such solutions lack sufficient stability and are described as bleaching photographic elements containing silver chloride emulsions only.
Other concerns with peroxide bleaching solutions include the potential for blistering in the element caused by the breakdown of peroxide into water and oxygen. This problem is referred to as vesiculation.
Other than vesiculation, there are similar problems associated with persulfate bleaching solutions. A rehalogenating persulfate bleaching solution needs a catalyst of some type, and the most commonly used catalyst is a thiol. As noted above, however, thiols are undesirable because of their odors.
There remains a need, therefore, for highly efficient peracid bleaching solutions which do not suffer from the problems noted above, that is, they are rehalogenating, stable, and useful for a variety of photographic elements, and lack objectionable odors.